NASA's Mars Global Surveyor has discovered surprising
evidence of past movement of the Martian crust, further evidence
that ancient Mars was a more dynamic, Earth-like planet than it
is today.

Scientists using the spacecraft's magnetometer have
discovered banded patterns of magnetic fields on the Martian
surface. The adjacent magnetic bands point in opposite
directions, giving these invisible stripes a striking similarity
to patterns seen in the crust of Earth's sea floors. On the
Earth, the sea floor spreads apart slowly at mid-oceanic ridges
as new crust flows up from Earth's hot interior. Meanwhile, the
direction of Earth's magnetic field reverses occasionally,
resulting in alternating stripes in the new crust that carry a
fossil record of the past hundreds of million years of Earth's
magnetic history, a finding that validated the once-controversial
theory of plate tectonics.

"The discovery of this pattern on Mars could
revolutionize current thinking of the red planet's evolution,"
said Dr. Jack Connerney of NASA's Goddard Space Flight Center,
Greenbelt, MD, an investigator on the Global Surveyor's
magnetometer team. "If the bands on Mars are an imprint of
crustal spreading, they are a relic of an early era of plate
tectonics on Mars. However, unlike on Earth, the implied plate
tectonic activity on Mars is most likely extinct."

Alternate explanations for the banded structure may
involve the fracturing and breakup of an ancient, uniformly
magnetized crust due to volcanic activity or tectonic stresses
from the rise and fall of neighboring terrain.

"Imagine a thin coat of dried paint on a balloon, where the
paint is the crust of Mars," explained Dr. Mario Acuņa of
Goddard, principal investigator on the Global Surveyor
magnetometer. "If we inflate the balloon further, cracks can
develop in the paint, and the edges of the cracks will
automatically have opposite polarities, because nature does not
allow there to be a positive pole without a negative
counterpart."

Peer-reviewed research based on the observations will
be published in the April 30 issue of the journal Science.

The observations of the so-called magnetic stripes were
made possible because of Mars Global Surveyor's special
aerobraking orbit. This process of dipping into the upper
atmosphere of Mars to gradually shape the probe's orbit into a
circle was extended due to a problem with a solar panel on the
spacecraft. The lowest point of each elliptically shaped orbit
curved below the planet's ionosphere, allowing the magnetometer
to obtain better-than-planned regional measurements of Mars.

"At its nominal orbit more than 320 kilometers (200
miles) high, the instruments face too much magnetic interference,
and they do not have the resolution to detect these features,"
Acuņa noted. "We began with misfortune, and ended up winning the
lottery."

The bands of magnetized crust apparently formed in the
distant past when Mars had an active dynamo, or hot core of
molten metal, which generated a global magnetic field. Mars was
geologically active, with molten rock rising from below cooling
at the surface and forming new crust. As the new crust
solidified, the magnetic field that permeated the rock was
"frozen" in the crust. Periodically, conditions in the dynamo
changed and the global magnetic field reversed direction. The
oppositely directed magnetic field was then frozen into newer
crust.

"Like a Martian tape recorder, the crust has preserved
a fossil record of the magnetic field directions that prevailed
at different times in the ancient past," Connerney said. When
the planet's hot core cooled, the dynamo ceased and the global
magnetic field of Mars vanished. However, a record of the
magnetic field was preserved in the crust and detected by the
Global Surveyor instrument.

The mission's map of Martian magnetic regions may help
solve another mystery -- the origin of a striking difference in
appearance between the smooth, sparsely cratered northern
lowlands of Mars and the heavily cratered southern highlands. The
map reveals that the northern regions are largely free of
magnetism, indicating the northern crust formed after the dynamo
died.

"The dynamo likely died a few hundred million years after
Mars' formation. One possibility is that later asteroid impacts
followed by volcanic activity heated and shocked large areas of
the northern crust, obliterating any local magnetic fields and
smoothing the terrain," Acuņa said. "When the crust cooled,
there was no longer a global magnetic field to become frozen in
again."

The map also identifies an area in the southern
highlands as the oldest surviving unmodified crust on Mars. This
area on Mars is where the magnetic stripes are most prominent.
The bands are oriented approximately east-to-west and are about
160 kilometers (100 miles) wide and 965 kilometers (600 miles)
long, although the longest band stretches more than 1,930
kilometers (1,200 miles).

"The bands are wider than those on Earth, perhaps for a
couple of reasons," Connerney said. "The Martian crust could
have been generated at a greater rate, causing a given magnetic
field to be imprinted over a wider area before it reversed
direction. Second, the Martian magnetic field may have reversed
direction less frequently, which would have given more time for
any one field direction to imprint itself in the steadily moving
crust, resulting in wider bands.

"In order to call this pattern a crustal spreading
center like that observed in the mid-oceanic ridges on Earth, we
need to find a point of symmetry, where the pattern on one side
matches the pattern on the other. We have not yet found evidence
of this type of symmetry," Connerney added.

Graphics of the magnetometer data, other supporting
material and general information on the Global Surveyor mission
may be found on the Internet at: